1. Field of the Invention
The invention relates to a measuring head for in-line determination of the size of moving particles in transparent media, comprising a tube-shaped measuring head body which has a parallel-walled opening, open on one side, in the region of its front end, located in the measuring space, for holding an optical measuring point with two measuring windows, consisting of an illuminating device arranged in an opening wall and a light receiving device provided in the opposite opening wall, which is connected to an opto-electronic converter device, whereby the particle flow is located between the illuminating device and the light receiving device, with the use of a rinsing device for cleaning the optical measuring point, whereby the rinsing device consists of a rinsing medium source, a channel leading as far as the opening inside the tube-shaped measuring head body for delivery of the rinsing medium, and at least one discharge opening in the region of the opening.
2. The Prior Art
The area of application of the invention is contact-free determination of the size of particles, i.e. solid, liquid and/or gaseous particles that are located in flowing fluids or gases or that are moving in a transparent medium or in a vacuum itself. Examples are dispersed multi-phase flows, e.g. dust flows, suspension flows, or aerosol flows, whereby the particle size can be determined without sampling, at a high data rate, in technological processes of lesser or greater complexity.
According to DE 196 28 348 C1 and DE 298 04 156 U1, such a measuring head is already known. It consists of a tube-shaped measuring head body, which can be introduced into a medium that carries particles. At the end of the measuring head body, there is a parallel-walled opening that is open on one side, and which has an optical measuring point assigned to it. The latter has two measuring windows located opposite one another in the opening, whereby one measuring window protects an illuminating device, and the other measuring window protects a spatial-frequency filter arrangement as a light receiving arrangement, with an additional light-wave guiding element. This spatial-frequency filter arrangement is passed to an opto-electronic converter device on the output side, so that when particles pass through the measuring volume illuminated by the illuminating device, in the opening of the measuring head body, an output signal of an alternating voltage type can be evaluated to determine the particle velocity on the one hand, and an impulse signal can be evaluated to determine the particle size, on the other hand.
An analog measuring head with a different light receiving arrangement, which obtains the output signals on the basis of time measurements, is known from DE 199 11 654 C1.
In the practical use of these known measuring heads in different particle flows, it has been shown that relatively rapid wetting or fouling of the two measuring windows of the optical measuring point takes place, particularly in the presence of dusty and/or sticky particles. The measuring windows close up as a result of the deposit of such particles, so that the optical permeability of the measuring point deteriorates to an increasing degree. As a result, exact output signals can no longer be determined, i.e. undesirable measuring errors occur in the determination of the particle size.
It is true that it is already known, from DE 298 23 184 U1, to equip an optical measuring head for determining the velocity of flowing fluids with a rinsing device for cleaning the optical measuring point. In this connection, the goal is to prevent floating materials entrained by the flow, particularly light materials, with a more or less bulky shape, such as grass blades, leaves, etc., from getting stuck in an intake channel of a plate-shaped head body that contains the optical measuring point, i.e. to prevent damage to a knife-like edge of the intake channel, if necessary. In this connection, this rinsing device consists of a line that conducts a pressure medium, which opens out centered in the back opening region of the intake channel of the plate-shaped head body, i.e. seen in the flow direction, after the optical measuring point assigned to the intake channel, and opposite to the incoming flow direction.
This known rinsing device is not suitable for use in the measuring head for determining particle size according to the general type, since cleaning and constantly keeping the two measuring windows of the optical measuring point clear, particularly of deposited (correspondingly small) particles of the particle flow is not provided and also not possible. The rinsing jet, which is directed against the inflow direction, is weakened at the higher flow velocities of the particle flow that is usual in technical processes, in such a way that an effective cleaning effect in the stated sense cannot be achieved at the two measuring windows. Also, the relatively, large distance of the outflow opening of the rinsing device from the measuring windows has a negative effect on the cleaning effect. Furthermore, the known arrangement of the rinsing device causes a disturbance of the particle flow in the region of the optical measuring point, and this results in inaccurate measurement results with regard to the particle velocity.
Furthermore, a measuring device for determining the powder portion in a gas/powder stream is known from DE 44 40 417 A1, which can be used in a transport line that carries the said gas/powder stream. It consists of a light-permeable housing for the gas/powder stream, through which the beams of a beam transmitter to a beam receiver pass. An insert pipe for carrying the gas/powder stream is arranged within the housing. This pipe has passage openings in the region of the beam path. A pure gas stream that flows in the annular gap between the insert pipe and the housing is supposed to prevent deposits on the inside wall of the housing and on light guides.
This known device is relatively complicated and therefore expensive, particularly because of the provision of a pure gas flow that completely surrounds the gas/powder stream. Furthermore, the described measures for preventing deposits on a measuring head for determining particle size according to the general type cannot be used, since here the particle flow is not carried in a closed housing (of the measuring device). Accordingly, the arrangement of an annular gap for carrying a pure gas stream cannot be implemented. Furthermore, the gas stream that flows in the annular gap cannot prevent the deposition of deposits in the interior of the insertion pipe of the passage openings, so that incorrect measurement values cannot be precluded.
Furthermore, a measuring head of the general type is known from WO 87/05 108 A1. Here, the rinsing device assigned to the illuminating device and the light receiving device, for cleaning the optical measuring point, essentially consists of channels that run parallel to the surfaces of the lenses, in each instance, for delivery of the rinsing medium, which channels empty into bores for guiding the light beam, arranged perpendicular to the particle flow.
However, this known arrangement is connected with disadvantages in terms of flow technology and function.